1. Field of the Invention
The present invention relates generally to an apparatus and method for creating a block code in a mobile communication system, and in particular, to an apparatus and method for encoding a quality matching indicator (QMI) using a block code in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
In general, a CDMA mobile communication system (or an IMT-2000 system) transmits data frames of various services such as a voice service, an image service and a data service, using a single physical channel. Such service frames are multiplexed (combined) into one frame before transmission. In transmitting the multiplexed frame, a data rate of the respective data frames can be set to a different value in order to guarantee a quality of the respective services. When the respective service frames have different data rates, a transmitter must inform a receiver of the different data rates.
A quality matching indicator (QMI) serves to indicate information on the data rates of the respective service frames. It is important for the quality matching indicator to be correctly received at the receiver. Shown in FIG. 1 is an exemplary way to use the quality matching indicator.
FIG. 1 illustrates a structure of a forward link data traffic MAC channel (DTMACCH) for a packet data service in a mobile communication system supporting the high-speed data transmission. As illustrated, the minimum transmission unit of a physical layer for the packet data service is a 1,536-chip slot having duration of 1.25 msec. The data traffic MAC channel (DTMACCH) is comprised of a first channel (in-phase channel) and a second channel (quad-phase channel). The first channel, in-phase channel, is used as a QoS (or quality) matching indication channel, while the second channel, quad-phase channel, is used as a Walsh space indication subchannel (WSISCH) and a reverse activity indication subchannel (RAISCH). In one 1,536-chip slot, the WSISCH and the RAISCH have a 1,280 chip period and a 256-chip period, respectively. The WSISCH and the RAISCH are transmitted over the quad-phase channel of the DTMACCH after multiplexing.
FIG. 2 illustrates a transmitter for transmitting the quality matching indicator. Referring to FIG. 2, upon receiving a 7-bit quality matching indicator to be transmitted in one slot, a (24, 7) encoder 200 encodes the received 7 quality matching indicator bits and outputs 24 coded symbols having values of ‘0’ and ‘1’. A signal mapper 210 or symbol encoder then maps the coded symbols from the encoder 200 by mapping ‘0’ to ‘1’ and ‘1’ to ‘−1’, and thus outputs symbols having values of ‘1’ and ‘−1’. The 24 coded symbols output from the signal mapper 210 are provided to a multiplier 220 in a symbol unit, and at the same time, a Walsh cover (or Walsh code) of length 64 is provided to the multiplier 220. The multiplier 220 multiplies the respective symbols by the Walsh code of length 64. That is, the multiplier 220 outputs 64 chips upon receiving every symbol, thus outputting a total of 24×64=1,536 chips.
FIG. 3 illustrates a structure of a receiver operating in association with the transmitter of FIG. 2. Referring to FIG. 3, upon receiving an input signal, a symbol accumulator 320 despreads the received input signal by multiplying the signal by the same Walsh cover as used by the transmitter, and then accumulates the despread signal in a symbol unit. The symbols output from the symbol accumulator 320 are provided to a decoder 300 which corresponds to the encoder 200 of the transmitter. The decoder 300 decodes the symbols from the symbol accumulator 320 and outputs a 7-bit quality matching indicator.
As stated above, the quality matching indicator value is expressed with 7 bits. If the quality matching indicator, containing information required by the receiver in analyzing the respective service frames, has a transmission error, the receiver cannot correctly analyze the service frames. Therefore, the quality matching indicator should be encoded using an error correcting code such that even though a transmission error occurs during transmission of the quality matching indicator, the receiver can correct the transmission error.
In general, an error correction capability of binary linear codes is determined depending on a minimum distance between the binary linear codes. For the optimal code, the minimum distance dmin between the optimal binary linear codes is disclosed in a reference entitled “An Updated Table of Minimum-Distance Bounds for Binary Linear Codes” (A. E. Brouwer and Tom Verhoeff, IEEE Transactions on information Theory, VOL 39, NO. 2, MARCH 1993).
Considering that the quality matching indicator transmitted by the transmitter of FIG. 2 is a (24, 7) code having 7 input bits and 24 output coded symbols, the minimum distance between required optimal codes is dmin=10 as described in the above-stated reference document. If an encoder for encoding the quality matching indicator fails to satisfy the minimum distance dmin=10, an error rate of the quality matching indicator increases in the same channel environment. Therefore, an error of the quality matching indicator occurs, so that the receiver may incorrectly recognize the data rates of the service frames, causing an increase in frame error rate during demodulation of the service frames. Therefore, it is most important for an error correcting encoder for correcting the quality matching indicator to minimize the error rate.